1. Field of Invention
This invention relates to a method of forming an anti-corrosive film on magnesium metal and its alloys which is smooth, adherent and durable, and to the product resulting therefrom. In particular, the invention relates to a method comprising pretreatment of such metals with a solution of hydrofluoric acid to form a fluoromagnesium film on the metal, followed by electrolytic coating in an aqueous solution comprising an alkali metal silicate (or other suitable salts) and an alkali metal hydroxide.
2. The Prior Art
The protection of metallic surfaces, including magnesium and its alloys, against corrosion and chemical attack has heretofore received considerable attention. Some protection has been afforded to metals by coating them with organic films such as paints or enamels. Although these coatings are relatively resistant to chemical attacks, they are subject to degradation at high temperatures and exhibit poor adhesion to metallic substrates, particularly when temperature cycling is involved.
Numerous other methods of coating metals and their alloys with a protective inorganic surface have been proposed and disclosed in several patents. For example, it is well-known to provide aluminum with a protective coating by an electrolytic process involving anodization of the metal. However, the anodized surface of aluminum obtained by this process does not withstand attack by various acids or even weak alkalis.
U.S. Pat. No. 1,923,539 discloses the formation of protective coatings on magnesium, aluminum and their alloys by an electrolytic process in various solutions, use being made of alternating current at about 100 volts. The coatings produced, however, are not sufficiently resistant to attack by corrosive agents such as strong acids and strong alkalis. Additionally, they lack the hardness and durability required in numerous applications.
Czechoslovakian Pat. No. 104,927 issued to Hradcovsky et al. discloses electrolytic coating of aluminum in which the electrolyte is an aqueous solution comprising sodium or potassium silicate and a hardener such as ammonium molybdate. The resulting silicate coating is highly porous and exhibits a weak breakdown voltage.
U.S. Pat. No. 3,832,293 issued to Hradcovsky et al. discloses a process for providing a durable coating on so-called "rectifier" metals such a aluminum and magnesium by an electrolytic process which comprises immersing the metal in an aqueous bath comprising an alkali metal silicate, an alkali metal hydroxide and an oxyacid catalyst, and imposing a potential difference of at least 220 volts between the metal and the bath to deposit a silicate coating on the immersed surface of the metal.
A more recent U.S. Pat. No. 3,834,999 to Hardcovsky et al. discloses another process for electrolytic coating of rectifier metals, comprising immersing the metal in a strongly alkaline bath containing an alkali metal hydroxide and at least one anion such as tungstate, phosphate, arsenate, stannate, stibnate, molybdate borate, chromate and dichromates, alone or in combination, and carbonate, in combination only. The process involves coagulation of colloids at the surface of the metal to produce glassy, adherent and corrosion-resistant coating.
Although the various methods described in the above-identified United States patents produce coatings which are superior to the porous coatings made by the method disclosed in the Czechoslovakian patent, the methods themselves suffer from one or more deficiencies. For example, substantial electrical potential is usually required to discharge visible spark at the metal surface and a relatively long period of time is frequently necessary to build up a uniform film of suitable thickness. Additionally, some of the electrolytes, particularly those containing sodium silicate, are unstable, with resulting precipitation and change in the electrolyte concentration. Consequently, the bath may not be reused over any substantial period of time.
The ionic conductivities of the coatings produced on magnesium and its alloys by the prior art processes do not usually satisfy the anti-corrosive standards required for these metals. Additionally, relatively long period of time (30 to 60 minutes) is usually required to form a satisfactory coating.
Thus, there is still a need for an effective process for coating magnesium and its alloys with a hard uniform, smooth, adherent and corrosion-resistant coating which can withstand attack by strong acids and strong alkalis.